


Because I Trust You

by WrittenKinzy18



Category: Undertale (Video Game)
Genre: Bromance, EXP and LOVE (Undertale), Friendship, Gaster Blasters, Gen, Heart-to-Heart, Judge Sans (Undertale), Judgement, No More Resets (Undertale), No one can convince me otherwise, Platonic Relationship, Post-Undertale Pacifist Route, Post-Undertale Pacifist Route - "I want to stay with you.", References to Undertale Genocide Route, Sans (Undertale) Remembers Resets, Sparring, Undertale Monsters on the Surface, non violent fighting, references to undertale resets, secret reveal, undyne and sans totally have a bromance
Language: English
Status: Completed
Published: 2020-12-04
Updated: 2020-12-04
Packaged: 2021-03-10 04:54:41
Rating: Teen And Up Audiences
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 6,841
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/27868750
Author URL: https://archiveofourown.org/users/WrittenKinzy18/pseuds/WrittenKinzy18
Summary: Sans' family is worried for his safety now that they're on the surface. Undyne proposes she trains him once a week to get his stats up.Sans proposes a new deal.
Relationships: Sans & Undyne (Undertale)
Comments: 27
Kudos: 255





	Because I Trust You

**Author's Note:**

> This has been an idea on my mind for a few weeks that I really hadn't planned on finishing so quickly. I wanted to do a second part to the Winged Multiverse first, but oh well this apparently took priority somehow. ᕕ( ᐛ )ᕗ
> 
> T rating cuz I'm a sailor and projected my cursing tendencies onto these two.

“Ya gonna tell me why we’re here now, or am I just gonna have to guess until you do?”

“I’ll tell ya when we get there!”

“Guessing it is. Are we having a tea party?”  
  


“I- what? No!”

“Is this some evil plot to assassinate me in the woods? Away from the public and a prime area to bury my remains and kick about my dust? If so, this is really cliché, and I’m kinda disappointed. If you’re gonna kill me, at least be original about it.”

“No! Shut up! We’re almost-!”

“Ah, I get it. Leading me to a secluded place to profess your _Undyne_ love to me. I’m terribly sorry, but I’m gonna have to say no. I could never do that to Alphys-“

_“NO!_ Gross!” Sans snickered as a stick was suddenly thrown in his direction, ducking underneath it just in time. Undyne glared at him in annoyance, another stick held threateningly in her hands as if it were one of her spears. “We’re here to spar, dipshit!”

The skeleton blinked, then shook his head. “Nah, that’s definitely an on-the-spot excuse. A total coverup. I guessed right, didn’t I?”

“SANS!”

“Alright, alright.” He threw his hands up in surrender. “Don’t get your _Undies_ in a twist, Fish Sticks.” He eyed her chosen weapon. “Or should I say, Fish with a Stick?”

Thank the stars for his dodging ability. That stick would have jabbed him right in his good eye socket.

“So why, pray tell, are we sparring? I didn’t think I was your prime choice for a fighting-buddy. Thought for sure that was Papyrus’ job.”

Undyne took a breath to rid herself of any aggravation. She then stood up straight with her hands proudly on her hips and a wide, sharp grin on her face. “Papyrus came to me asking to help train you in self-defense! Because you’re a lazy, weak piece of shit that needs to up your stats before you get dusted from tripping on your own shoelaces!”

He raised a brow. “Wow. You certainly let loose a lot more when Tori isn’t around.” Not that he was to judge. He cursed and told much darker jokes when around the right audience; aka, not around Toriel or Papyrus. Sticking his hands into his jacket pocket, Sans took a moment to glance around his surroundings. Like he said, they were a long way from Ebott now, deep in the woods at the base of the mountain. They weren’t far from the trails, and it was easy to know directions with the mountain looming to the east, so getting home wouldn’t be an issue. Besides the fact that it was a long walk for his lazy bones. He’d much prefer to shortcut back, but he couldn’t exactly do that in front of Undyne.

Speaking of the crazy fish lady, she had led him to a fairly open clearing that had obviously seen better days. From the many scorch marks and busted trees, it was safe to assume this clearing was commonly used for her training anyway. Not surprising. They couldn’t exactly practice magic attacks at their local gym, after all.

Well, not yet at least. New rules and laws on magic use were still in the air.

“I’ll have you know, I only trip on my shoelaces when I’m especially drunk, and I haven’t dusted myself once. That I remember, at least.”

That grin fell from Undyne’s face, turning into an exasperated expression. “That’s not- UGH, you’re infuriating!”

“Daw, you love me, admit it.”

“Shut up.” The Captain crossed her arms and hit him with a serious stare. “Look, your brother is really worried about you, punk. The Surface ain’t the same as the Underground. Monsters are getting dusted left and right by hate groups the city can’t control, let alone the Guard. We all just want to be sure you can handle yourself alright.”

Sans smiled reassuringly. “Yeah, I kinda figured. But I promise I’m fine, really.”

“I don’t believe you! Your base stats are one. Your _HP_ is one. If a human or monster comes at you with even the slightest intent to harm, you’re dead.”

“Y’know stats aren’t everything, right?”

Undyne growled in frustration. She expected his resistance, but it was still annoying. “Yes, I _know_ you can dodge. But what are you going to do when someone sneaks up on you?! You can’t dodge forever, Sans! You can’t dodge multiple people and multiple attacks! You’ll either get taken by surprise or worn down within the minute because you do nothing to keep your stamina up!” That was more of an assumption on her part. She knew he was lazy and _likely_ easily tired out, but she’s also play-fought him enough at the house to know he had decent stamina anyway. Her point still stood, though. That was play fighting. This was serious. “God, how many times have we found you sleeping on a park bench?! You’re not exactly _hard_ to catch off guard!”

Wow. They really thought long and hard about this, then. They probably had a secret meeting to discuss his Dusting Likelihood on the Surface, if he knew his family well enough. And he did. Sans could almost see the poster sized graphs and statistics Alph surely brought to said meeting. He was almost touched if this wasn’t completely unnecessary.

He sighed, running a hand over his skull. “Undyne, really, as nice as this is-“

“No, you can’t talk your way out of this!” She summoned a spear in her hand.

“I’m trying to tell you-“

“I don’t want to hear your punny excuses! We are going to train here once a week until I am satisfied with your stats! For Papyrus’ sake and the rest of your family, please don’t lazy your way out of this.”

He hit her with a deadpanned stare.

“You know what I mean!”

“Oh stars…” He took a breath, lamenting over all he ketchup he could have been drinking right now. Instead he’s here, dealing with his families protective and caring tendencies. He wasn’t complaining about that by any means. Sans loved his little ragtag family, as weird as they were. What he _was_ complaining about was the fact they all saw him like a glass figurine. Breakable.

He was _far_ from breakable.

He briefly wondered if Frisk was present at this secret meeting. He couldn’t decide if they would have tried to ease their worries and ended up unsuccessful, or if they went along with it so they could tease him about his “training” later.

He had a sinking feeling it was the later. He taught the kid well.

Still, Sans refused to spend his time uselessly training his attacks with someone who was, honestly, either below him in skill or on par with it. He had half the mind to leave right now, just started walking back home, dodging Undyne’s protesting attacks until he either lost her or got far enough she gave up, but he knew he’d never hear the end of it. If this meetup with Undyne didn’t work, the others would pester him with their concern and new, probably worse ideas endlessly until he gave in. They were just scared for him, especially now that this was, technically, their happy ending.

Sans understood that fear all too much.

So, in the end, he relented.

Well. Sort of.

“I’m not trying to get out of it, Undies. If you’d let me finish, please?”

She paused, looking almost surprised at his lack of resistance, then nodded. The spear didn’t dissipate, her grip still tight and ready.

“Okay.” He shifted to the balls of his feet. “I was gonna make a deal with you. I know I’m not getting out of this, at least not without proving myself to you somehow. And I appreciate you guys caring about me — really, I do. But I’m telling the truth when I say I _can_ handle myself.” He held up a hand when she went to protest. “I know you don’t believe that, and I don’t expect you to. I…” The skeleton hesitated.

He knew he wasn’t getting out of this one without either fighting for real, or faking being exactly what they thought he was. Obviously, that last option was the least desirable, but the most realistic. But he really didn’t have the energy to play pretend for so long. However, technically, he wasn’t supposed to share anything about his abilities to anyone but the King. It was part of being the Royal Judge. You can keep an eye on people more efficiently when they trusted you and didn’t think you were judging their every action. It’s supposed to be a secret job for the strongest monster. It’s why he had so many connections and friends — the more people that knew that liked him, the more they let their guard down, and the easier it was to find who was good and who was bad.

Most were reported to the Guard to handle. Others were given to him. Depended on how high their LV was.

But… was the Judge even needed anymore?

Sure, he still got his checks from the King, and he still kept up with his job every time he went out to socialize, Judging every person he came across, but how essential was it to keep everything a secret anymore? It’s not like he could keep tabs on _everyone_ now that the whole Surface was open to them.

Then again, it’d be beneficial to at least check on everyone in Ebott…

Sans eyed Undyne, who was surprisingly patient while he thought about his options here.

Undyne was the Captain of the Royal Guard, even on the Surface. The Guard sort of merged with Ebott’s police force after a few months and mainly focused on monster affairs rather than human. Besides him, she was the closest to Asgore’s throne, and rightfully so. If there was anyone that had a truly righteous and genuine soul among monsters, it was her. She did everything for the sake of the people she protected and cared for and would put her life on the line because of it.

_Has_ put her life on the line because of it.

He may not be able to come clean to his own brother, but he was sure Asgore would understand telling Undyne.

“Alright, here’s the deal.” Sans put his hands back into his jacket pockets and stepped further into the clearing. The other monster raised an eyebrow but let him continue. “Whatever is said or whatever happens in this forest, stays in this forest, alright? You can’t tell anyone anything. Besides king Fluffybuns, of course.”

Undyne narrowed her eye in confusion. “What?”

“Just, promise me. For once, I’m being completely serious.”

Silence stretched out between the two as they held eye contact, both looking for any sort of untrustworthiness or deceit in the other. The only sounds came from the blowing of the wind and the singing of the birds in the trees around them. It was really a peaceful place out here. He’d have to come stargazing one night.

Eventually, the female tilted her head, still eying him suspiciously, and said, “Okay. Why?”

Tension he didn’t realize had built up in his shoulders suddenly relaxed. “Cool. ‘Kay, so, for the sake and sanity of my bro and the rest of the gang, I know we gotta keep doing… something. I don’t think I need any training, and you don’t believe me. So, I see only one solution here.” Sans raised a hand and gestured to himself then the Captain. “We have one battle, right here right now. No intent to hurt or kill, but otherwise no holding back. First to subdue or wear out the other one wins.” Undyne was looking more and more incredulous the more he spoke, but he continued anyway. “If you win, we do whatever you planned already. Weekly training, no complaints, no exceptions, whatever you want. I’ll even _try_ , no half-assing it.” He tilted his head down and stared up at her seriously. “But if _I_ win, we…” Sans huffed, shifting on his feet. “If I win, we still meet up weekly, to keep up appearances for the others. Make them _think_ we’re training, but really we just, I dunno, have some bro or gal time, whatever you want. Go to the movies, take a walk, get our nails done, somethin’. Maybe actually spar a bit if you really wanna.” He held out his hand. “That sound good?”

“Wha-? No!” Undyne threw her hands in the air. “I’d fucking dust you first shot! That does _not_ sound good! This is totally rigged against me!”

Rolling his eyelights, Sans kept his hand out and persisted. “You won’t dust me. I’m tougher than I look. If it makes you feel any better, I slept a good while before coming out here. Technically, my HP is eleven right now. As long as you don’t have any _real_ harmful intentions in your hits, I’ll be good to take a couple shots.” He pushed his hand out further, giving her a deadpanned stare. “Deal?”

The taller monster stared at his hand for a moment before turning that piercing gaze back to his. “This isn’t some kinda trick? You’re not gonna prank me somehow and bend the rules to make me loose or anything?”

“Undies, if you don’t shake my hand, I’m just gonna leave and you all can deal with the anxiety of having my ass out on the streets ‘untrained’ or whatever.”

“Pfft, like you’d get very far.”

He raised a boney brow. “I have a lot of secrets up my sleeves, Fish Sticks. Disappearing without a trace is one of them.” A grin came back to his face as he winked at her. “Trust me when I say I know a shortcut to Grillby’s from here.”

Growling, the fish monster stomped forward, magic spear still sparkling in her grasp as she grabbed his hand in a tight handshake. “Fine. Deal.” A slight smirk crossed her sharp tooth grin. “For a lazy pile of bones, you sure seem confident to fight the captain of the Guard.”

Sans tightened his grip to match hers. “Eh, I’ve fought a few people once ‘er twice.”

“I seriously doubt that.”

“I had sentry posts, didn’t I?”

“Yeah, and all you did was nap at them!”

He shrugged. “I may have napped most the time, but I was still aware of who was around me.”

She rolled her eyes, letting go of his hand to playfully shove his shoulder. “Yeah, in your dreams. Literally.”

Snorting, Sans put his hand back in his pocket. “Suit yourself, Undie.”

A crazed look came to Undyne’s one good eye. “You ready to get pummeled, punk?!”

“Yeah, sure. Go ahead and take the first shot.”

Blue spear materialized in the air around the Captain, each one sparking with enthusiastic energy to match their creator. The blue glow reflected in the eye not covered by an eyepatch as she pointed forward, firing them his way. “This’ll be over fast!”

She was still within arm’s reach, so Sans didn’t have much time to hop out of the way. He just barely managed to avoid each spear, leaving a trail of them impaled in the dirt in front of him. The skeleton merely grinned at the attempt, sidestepping a yellow one that tried to shoot him from behind. “Yeah, you might be right there.” A barrage of simple bones flickered into existence around his head in a similar fashion. When he raised his hand to command them, it glowed a cyan blue.

As expected, Undyne easily dodged the oncoming bones, meeting many with her own attacks. When she found a moment between two rib bones, she launched the spear in her hands at the skeleton.

For a moment, it looked as if he wasn’t going to dodge like usual. A slight spark of fear flashed in her soul, wondering uselessly if she put too much intent into that attack, when suddenly Sans flickered out of her view.

The large spear struck a tree far behind it.

Undyne blinked at the empty spot in confusion, almost too distracted to miss the assortment of bones that shot up under her feet. She barely stumbled out of the way.

“I told you,” a voice said behind her. She whipped around, finding the smaller monster standing safely on the far side of the clearing, casually leaning against a tree. “I have a talent in _disappearing without a trace_.” He winked at her, lifting his hand to send a barrage of bones up once again from the ground below her.

The shock drained from her face, replaced with an excited grin. “Oh, you’ve _gotta_ teach me how to do that!” She fired a larger than normal spear in his direction, only getting more giddy when he teleported just out of its range of damage.

“Don’t think I can do that, Undies,” he shrugged, ducking under another strike. “Kind of a Sans Only thing.”

“Are there any other _Sans Only things_ I should be worried about?”

That carefree smile only grew larger. “You don’t know the half of it.” Both his white eyelights went out, replaced by a flashing blue and yellow ring in his left eye. He snapped his fingers.

Undyne almost missed the whirring sound of something powering up behind her. Her mind kicked into gear when she heard it fire off and she jumped to the side, narrowly avoiding a large beam of energy that shot across the clearing. When she scrambled to her feet again, her wide yellow gaze shot between the completely scorched trees and the smoking dragon-like skull that floated just behind where she once stood.

As quickly as it came, it dissipated like monster dust.

“You- wh-“ Undyne fumbled for words while the comedian simply snickered at her reaction. Her hand shot up to grip at her hair. “WHAT THE FUCK WAS THAT?!”

Grinning amusedly, Sans snapped again. Three more skulls came to life behind him, much smaller than the first one. Each one opened their jaws wide, mandibles splitting open, allowing a sphere of pure white and blue power to charge within their maws. That energy was then cradled between their sharp teeth, waiting to fire. The skeleton comedian took a few steps away from the tree line to better face her, each skull trailing behind him in their original formation. “They’re called Gaster Blasters, and I guess you can say I’m their _master_.”

“DON’T YOU MAKE JOKES WHILE WE’RE FIGHTING!” More spears crackled around her form. Just in case, she brought out her blue shield as well. It wasn’t usually something she used in battle, but she had a feeling it’d be necessary here.

“Or what? Ya gonna _krill_ me?”

“NGAHHH!”

All the attacks she sent his way were immediately destroyed by a single shot of Sans’ blasters, even the few yellow ones she sent to confuse him. It made her faulter for a moment, but she quickly regained that passion.

“You’re attacks might look cool as shit!” she shouted, raising her shield to block the group of femur and humerus bones fired her way. “But I know your stats are still too low to cause any _real_ damage!”

Sans simply shrugged, bringing out fresh blasters when the others ran out of juice. “That may be so, but I’ve found ways around that.”

“What does that- OW, SHIT!” A fibula bone had shot out of the dirt and stabbed through her foot. It didn’t hurt all too much, but a feeling she wasn’t expecting suddenly coated her body and soul.

Her HP was _draining._

Typically, a monster’s attack struck and did a set amount of damage. If removed right, no further harm would come of it. Usually, that just meant dispelling the attack immediately after it hit. If a monster was cruel enough, they could leave it in hopes that their opponent pulling it out would cause more damage. Undyne was prepared for such events. She’d been trained in removing even the stickiest of attacks quickly and efficiently without doing any more damage. She was _not_ prepared for this. Sans’ simple bone attack somehow _continued_ to harm her. It was like some sort of dull poisoning, dealing one damage to her HP at a time, rapidly. Wincing, Undyne propped up the shield on her shoulder as protection and yanked the bone out of her foot.

The monster threw the offending bone away and checked her HP.

In one shot, he managed to take out a _quarter_ of her health.

“Holy shit,” she muttered disbelievingly.

She started to wonder what she got herself into.

Another beam charged up and shot at her shield. Undyne held it up strong, her feet scraping along the ground as its power pushed her.

“You doin’ alright over there, Undies?” she heard the skeleton tease. Grumbling, the fish monster debated her next move.

Sending another wave of spears was useless. She’s wrestled around with Sans enough to know he could dodge for a while if he really wanted to. It’d wear him out, she was sure, but it would also wear _her_ out. It would be a race of stamina, and with his attacks, she wasn’t really sure anymore if she’d beat him there.

Another option was close quarter combat. If she could get in close proximity, she could fight him hand to hand, or even go all anime and battle it out like a sword fight, spear on bone. Undyne figured he didn’t know much outside of distance attacks, as it would heighten his chances of getting hit if his enemy was too close. However, she definitely wouldn’t be able to get all that close with his Blasters. Not to mention, he’d likely just teleport away.

Sans had already long proved himself to be strong enough on his own. She could call the fight off right now.

But when has Undyne ever admitted defeat so easily? _Never._ She’d either beat this funnyman or go down kicking.

Another blast hit her shield, making her wince, and putting her mind back on track.

She can’t get close, but she can’t keep attacking from afar either. He’ll just continue to dodge all of her advances. The best idea would be to keep him from moving at all.

That grin spread across her face once again. This day was a lot more fun than she thought it would be. Undyne had expected she’d either end up chasing a refusing skeleton all day, or teaching him extremely weak and simple beginner attacks. Who knew _Sans_ of all people would be a good sparring buddy?

As soon as the laser beam died down again, Undyne stood up to face the skeleton. Throwing a hand out, Undyne grabbed his Soul and turned it green.

A slight flash of panic crossed Sans’ mind, but he squashed it quickly.

“TRY DODGING NOW, PUNK!” she yelled, firing another group of spears.

Sans’ eye sockets narrowed. He tried to teleport but only ended up flickering in the same position. Thinking quickly, he raised a large ribcage around himself, using a spare bone to block any attacks that slipped between those rib bones. The spears hit from all sides, gaining a triumphant shout from his opponent. When the sound of magic on bone cut away, he raised both hands and _pushed_ half of that bone shield at Undyne like a moving wall.

When she nimbly flipped over his attack, she took the opportunity to throw up another spear and kick it above her head. A blaster immediately formed in front of him, taking the spear through its skull and quickly firing in retaliation before disappearing again. Sans gave her an exasperated look.

“You’ve been wanting to do that flip-kick-midair thing for a while, haven’t ya?”

“HELL YEAH I HAVE!”

“Stars, you watch too much anime.” Still, a smile crossed his face. She returned it, yanking a discarded spear from the ground and pointing it at him.

“Ya give up now, punk?!”

He tilted his skull. “Why do’ya think that?”

“Ya can’t dodge anymore! You’re stuck right where I want’cha!”

“Ah, well,” he held up his hand, the familiar ring of energy igniting in his left socket, “guess I’ll make this even then.”

Suddenly, gravity weighed heavily on Undyne as her soul turned blue. She snorted, raising an eyebrow at the skeleton. “Blue magic? Y’know Paps uses that all the time, right? I know how to deal with this easy!”

“Yeah, but my bro’s still a beginner.”

He didn’t give her a chance to respond. In one swift movement, he threw his arm to the left, taking Undyne with it.

Said monster shouted in surprise when suddenly her center of gravity was off balance and she was falling sideways towards the edge of the clearing. She barely caught herself on the trunk of an already damaged tree when the skeleton shot his arm the other way, across his chest, and had her falling the complete opposite direction.

“WHAT THE FU-?!” She slammed gracelessly into another tree, wincing at the significant damage that took on her HP. This was definitely a side of Blue magic she didn’t know about. Usually, it was just used to make it harder to avoid attacks. The Captain had no idea it could be used to cause _actual_ damage, let alone manipulate gravity!

When Sans next threw her in a direction, she spun around long enough to see an array of bone attacks sticking out of the trees she was falling towards, waiting for her to hit them like some messed up bed of nails. Quickly, she summoned another spear and dug it into the dirt just before she was impaled. _Multiple times_.

Apparently, in her sudden disorientation, her Green hold on Sans’ Soul was relinquished. He walked up to her position with a large grin.

Wasn’t this a weird position they were in? Undyne, holding on to her spear, unnaturally parallel with the ground while the lazy bastard stood casually above her.

She glared his way.

“You good there, Undies?” he asked, hands back in his jacket pockets but his eyelight still lit with flashing cyan and yellow power.

“Shut up and let me down!”

“You sure? You seem like you’re havin’ fun _hanging around._ ”

“SANS!”

“Hmm.” The skeleton tilted his head innocently. “I dunno. Ya gonna admit to defeat?”

“What?!” Undyne growled, narrowing her gaze. “NEVER!” A few spears shot at him from her sides. Each was easily blocked by a couple bones.

“You sure? ‘Cause I can do this all night.” As if to emphasize his point, Sans sat on the floor, crossing his legs and leaning back with a lazy smile. He even closed his eyes and faced the setting sun, the fucking dick.

She sent another wave of attacks at him. Somehow, without even looking, all of them were blocked once again. “HOW THE HELL ARE YOU DOING THAT!?” The Captain squirmed and kicked out in frustration, bouncing on the spear. She contemplated letting go, but one quick look at the bones still awaiting her on the tree below convinced her otherwise. That’d definitely weaken her enough to quit the battle, anyway.

She was in a bit of a tight spot here.

“When you decide to call it quits, let _minnow_.”

God, and it just gets _worse._

“Nice place you picked, by the way. Quiet, secluded, pretty. Can’t get much _betta_ than this.”

“Ugh, god, just kill me now.”

“Don’t you mean _cod?_ ”

“That’s so overused, asshole!”

“Sorry. Had to take the oppur- _tuna_ -ty.”

“Fucking hell, if I say you win will you _stop with the fish puns?!_ ”

“Ah, I thought I was _piranha_ roll.”

“God dammit. If I aim right, would those bones kill me?”

Sans laughed heartily, raising a glowing blue hand and snapping his fingers. With that, the bones below her faded away and the Blue hold on her soul released.

She faceplanted into the grass and dirt, hands still awkwardly holding onto her spear. With a groan, she released her magic as well, letting her arms flop to the grass.

Raising an eyebrow, Sans nudged her arms with his foot. “We good, Fish Sticks?”

He received a grumble in reply.

“Not really an answer, but I can try and translate. ‘Oh, Sans, you’re so amazing. How could I ever profess my love to you, knowing you are so much cooler than I?’”

Undyne snorted, lifting her face from the ground to give him an exhausted grin. “You’re really milkin’ this ‘confession’ shit, asshole.”

“Hey, milking shitty jokes for all they’re worth is what I do best.”

“God, ain’t that the truth.” When she sat up on her elbows, her yellow gaze took in the amount of destruction that laid out around them. “How the hell are you so lazy but packing so much raw power?! Like, you’re tiny and literally all bones. Where do you even store all that magic?!”

“Do you want to joke answer, or the serious one?”

It took her a second to get the implication. “Gross!” Despite her outburst, she laughed and punched him in the leg. “The serious one. I don’t wanna know what disgusting bullshit you’d come up with.”

With a chuckle, Sans tapped the left side of his head. “It’s concentrated mostly in my left eye, after it leaves my Soul.”

“So that’s why it’s all flashy flashy when you’re fighting.”

It was much more complicated than that but… “Eh, more or less.”

Undyne laid down again, resting her chin on her forearms. “That was seriously impressive, dude. Like, I’ve never seen magic like that. You’re a fucking powerhouse disguised as a helpless asshole.”

He shrugged, turning his gaze back to the sky. “I mean, kinda have to be powerful to be the Judge, so.” He found himself once again marveling at the warm colors it turned as the sun set out of his sight. A warm gold faded into an almost pink color, broken only by the small dotting of similarly colored clouds. The stars weren’t out yet, but he still found himself mesmerized. He’d always thought the sky was only blue or black when he was Underground, as that’s all he saw in pictures and books. It was beautiful to see how colorful it could actually be in real life.

His view was blocked by a suddenly baffled fish monster.

“YOU’RE THE WHAT?!”

“Shhhh,” he muttered, raising a hand to Undyne’s mouth. She was practically in his face all too suddenly, and much too loud. “I’m enjoying the view, Undies.”

“No no no!” She shoved his hand out of the way. “You don’t get to drop a _bomb_ like that and expect me to just _ignore it!_ ”

He turned his eyelights down towards her, finding great amusement in her completely baffled expression.

“Like I said, Fish Sticks. Nothing leaves this forest, yeah?”

“Sans, this is _huge!_ Like, not only you fucking _telling me so casually_ , but also, you- The Royal Judge is a higher position than _I have!_ God, Sans, you could have just fucking _ordered me_ not to train you thirty minutes ago!”

He snorted. “Would you have honestly believed me thirty minutes ago?”

“I-!” The Captain paused, then retreated a bit and rubbed the back of her neck. “I… no, probably not.”

“Exactly.”

“You could’ve like, called Asgore or something.” She ran a hand down her face, expression pinched in frustration. “Dammit, this actually explains a lot. Your unbelievable popularity, how buddy-buddy you are with Asgore, the fact that you had a house that was way too spendy for a couple sentry station salaries, not to mention how hesitant Asgore was when we all brought up this idea of me training you.”

“Heh. So you _did_ have a meeting. That’s hilarious.”

She pointed a very serious look at him again. “The Royal Judge is a huge deal, and it’s supposed to be _extremely_ secret. Like, only revealed if _absolutely_ necessary, secret. Why the hell are you telling _me_?”

He shrugged. “Dunno.”

“That’s _not_ a good answer.”

Sighing, Sans turned away from the darkening sky above them. He leaned toward his friend to better address her. “I know it’s not something I’m supposed to be spouting willy nilly. Trust me. I’ve had this gig for quite a few years now.” Give or take a few hundred Resets as well. Truthfully, he’s had similar exchanges with Undyne before, when he thought getting the Captain to fight with him would help solve their genocidal flower issue. Those weren’t exactly the same as right now, considering they had a life or death situation to try and figure out then. Surprise surprise, it didn’t work anyway. But that was a whole other conversation for a different, hopefully nonexistent day. “I’m not just telling you on a whim.”

“Then why?”

Sans stared at her for a moment. He closed his eye sockets and took a breath. When they opened again, half lidded, his eyelights were gone. “The job of the Royal Judge is to Judge every monster and human they cross. I take in everything I see to make an accurate Judgement on whether they are good or bad. That means their words, actions, stats, and their LV.” Undyne straightened up at that. LV wasn’t an easily accessible stat to see on other Souls. If one murdered enough and had too much LV, they gave off an uncomfortable or dangerous aura, but most people never reach that point. Most only kill a few here and there rather than full on genocides. Having any LV wasn’t only a huge mark on a monster’s life, but having too much was dangerous, both for them and the people around them.

The fact that Sans could see it so easily was unusual and amazing.

“So you can tell a murderer from their LV, right?”

“Not exactly.” At her confused look, Sans continued, “Take yourself, for example. I’ve Judged you a million times since meeting you. You are one of the most loyal and trustworthy monsters I know. You’d give everything to save a single person and fight for what you believe. Essentially, you’re justice personified in a monster body.” Undyne looked genuinely surprised and flattered at his complement. He didn’t give those out often. “But, you have an LV of four.”

Suddenly, she looked horrified.

“Don’t freak out about that.”

“Don’t freak- LV is for _murderers!_ ”

Sans gave her a confused look. “What? No. LV is a person’s Level of Violence.”

“Yeah! It’s-!”

“It measures someone’s will to hurt others.” This didn’t calm her in the slightest. Stars, what were monsters taught these days? “It isn’t _just_ increased with killing and EXP. And even when it is, a person isn’t _immediately_ evil if they’ve killed someone. It could have been self-defense, or with a genuine motive behind it, like, say, freeing all of monster kind?”

Undyne blinked at him, furrowing her brows. “You gain LV through protective acts like that as well? I thought it was just… killing for the sake of killing — _wanting_ to hurt someone.”

He smiled. “And that’s why I know you’re a good person. You don’t _want_ to hurt anyone. You just know you have to. But, to answer your question, yes, you gain EXP and LV through killing of any kind. You can also gain it through mere intent. You don’t necessarily have to have killed someone to have any kind of LV. It’s a measurement of your likeliness to harm, after all. You can harm through more than just physically wounding others. Bullying, abuse, in the middle of a battle, even just getting extremely angry at someone for getting your order wrong — it all can increase your LV. That also means it can _decrease_ as well. It’s a surprisingly fluctuating stat.”

The other took a while to take this all in, then nodded in understanding. “Alright. So, you look at LV and make a judgement based on whether they harmed for good or bad reasons. What does this have to do with telling me all this?”

“Everything I just told you. I Judged you a long time ago and continue to do so every time we see each other. It’s habit, really. You’re a good monster, Undies. I trust ya with my brother’s life every day. I think I can trust ya with this.” He shrugged. “Plus, it’d be nice to _not_ be treated like a porcelain vase every so often.” She winced at that.

“Ah, yeah… I can see how that’d get… annoying.”

“Don’t worry ‘bout that. You and the kid are the only ones that treat me normally in games and shit. I appreciate it.” He paused, tilting his head. “Well, until today. You were totally going easy on me at the beginning, but that’s fine. Kinda expected it, after you said you’d had a talk with Paps.”

“Sorry, man. I just didn’t wanna be responsible for dusting his brother, y’know? Like, I was supposed to come here to train ya, not kill ya.” The Captain scratched her chin a bit. “’Till you whipped out your dragon murder skulls and fuckin’ kicked my _ass_.”

“Pff, I _hardly_ kicked your ass. This was a short play fight. I mean,” he winked slyly at her, “I didn’t wanna be responsible for dusting Alph’s girlfriend, y’know?”

Snorting, Undyne shoved his shoulder playfully. “Shithead! Don’t use my words against me!”

The two laughed together for a moment before descending into a comfortable silence. Sans continued to watch the sky grow darker by the minute, patiently waiting for the galaxy to show itself without the sun’s or the city’s interference. Undyne laid down beside his crossed legs, staring up with him.

“So…”

“So what?”

“What do we do now?”

He looked down at her questioningly. “What do you mean?”

“Well, your proved you don’t need my lessons. How do we convince the others as well without spilling the whole Judge thing.”

He blinked. “Uh, we don’t?”

Undyne looked at him incredulously. “What?”

“Don’t try to weasel yourself outta our deal. I won fair and square. We pretend to do your little training lessons every week.”

“You were serious? Why though?”

“Well first off, it put’s their minds at ease without them getting all suspicious and sticking their noses where they shouldn’t be.” Sans looked back up, eyelights immediately fixating on the first little spark of light that finally showed itself. “And second of all, we’ve known each other for who knows how long and have been living under the same roof since we got to the Surface, since none of us can afford a house on our own yet. Yet, never once have we really hung out. I dunno ‘bout you, but I find that pretty insulting. As the clear winner of our very first battle, I demand weekly hangout sessions.”

Undyne didn’t answer him straight away. She stared at him for a while, watching his smile grow wider with each star that came into existence.

He was right, she realized. The most they ever hung out was either with everyone else, or to playfully argue and fight in the house. She considered him a dear friend but didn’t actually take any time to personally get to know him. That… was rather upsetting, now that she thought of it.

“Alright,” she said softly, a giant smile gracing her lips. “Then we meet up again at the same time next week. I don’t know how you plan on hiding this, though. We can’t exactly hang out in town where they’d easily see us. That means no Grillby’s.”

“Don’t break my heart, Undies.” Sans snickered. “But yeah, I kinda figured. We could either come back here and do whatever, or I can teleport us to the next city over for a day.” He glanced down at her briefly. “Heard they’re gonna have a fair pretty soon. If the rest of the family doesn’t wanna go, we could sneak out and ride all their worst, gut-spilling rides.”

“Oh, _fuck_ yeah.” When her newly found excitement wore down, she registered what he said before. “Wait, you can teleport _with_ someone?!”

“Well yeah. Why do you think I’m not all that in a hurry to leave before it gets dark? We can stay out here as long as we want and still be home before anyone went to sleep.”

“That’s so cool. And it explains _so much._ ”

“Yeah, I’m gonna miss the Undyne prank potential, but I guess it was worth it.”

The two stayed out long into the night, casually chatting and watching the sky. It got a little too dark eventually, so Sans conjured a small blaster to serve as a light between the two. (Undyne fawned over how simultaneously awesome and adorable it was and insisted on being the one to hold it, despite it’s floating ability.) When they eventually got worried texts from Toriel and Frisk, they eventually decided it was time to head back home.

If the two were a bit closer than before, no one commented on it.

**Author's Note:**

> When the time came around the next week, Undyne forced Sans to go to a nail salon. She herself didn’t get anything done, but she wanted to see if he’d live up to his words of “bro or gal time”. Sans, of course, didn’t protest at all to sitting around and doing absolutely nothing for thirty minutes. He didn’t technically have nails, but they treated his metacarpals pretty much the same way. He had blue "nails" the for the next few weeks. 
> 
> I just felt a massive need to write some Undyne&Sans bromance. I feel like they’d be BFFs, man. 
> 
> Plus, the whole Secret Judge headcanon? Fucking love it.
> 
> Come yell at me if ya wanna.  
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